The electric parts and auxiliary machines of the car and motors of industrial machines are demanded to be compact and have high performance and output year by year. Thus the use conditions thereof have become strict. Rolling bearings are used therefor. Grease is mainly used to lubricate them.
Owing to a recent growing demand for development of small cars, lightweight cars, and the improvement of silence, attempts are being made to manufacture smaller and more lightweight electric parts and auxiliary machine parts of cars and make the engine room airtight. On the other hand, there is a growing demand for a high output and a high efficiency for the performance of various apparatuses of the car. Thus the present tendency is to compensate the miniaturization-caused reduction of the output of the electric parts and auxiliary machines of the car provided inside the engine room by rotating them at high speeds.
As examples of rolling bearings for use in the electric parts and auxiliary machines of the car, a rolling bearing for use in the fan coupling apparatus, a rolling bearing for use in the alternator, a rolling bearing for use in the idler pulley are outlined below.
Regarding the fan coupling apparatus, a viscous fluid is sealed inside it. A housing having an air-feeding fan mounted on its peripheral surface is coupled to a rotor directly connected to the engine through a bearing. By utilizing the shear resistance of the viscous fluid which increases and decreases in response to an atmospheric temperature, the fan-coupling apparatus controls the amount of a driving torque transmitted from the engine and the number of rotations of the fan, thereby feeding optimum air corresponding to the temperature of the engine.
Therefore the rolling bearing for use in the fan-coupling apparatus is demanded to have a high heat resistance, grease-sealing performance, and durability so that the rolling bearing withstands a high rotational speed change in the range from 1000 rpm to 10000 rpm in dependence on a fluctuation of the temperature of the engine and in addition withstands very strict environment in which it is driven in summer at a high speed not less than 10000 rpm at a high temperature not less than 180° C.
The alternator for use in the car receives the rotation of the engine and generates electricity, thus supplies an electric power to an electric load of the car and has a function of charging a battery. Therefore the rolling bearing for use in the alternator is demanded to have a high heat resistance, grease-sealing performance, and durability so that the rolling bearing withstands a very strict environment in which it is rotated at a high speed not less than 10000 rpm at a high temperature not less than 180° C.
The idler pulley of the car is used as a belt tensioning part for the driving belt transmitting the engine power to the electric auxiliary machine thereof. The idler pulley has the function of a pulley for imparting a tensile force to the driving belt when the distance between rotating shafts is fixed. The idler pulley has another function of an idler used to change the travel direction of the belt or preventing interference with obstacles to thereby reduce the volume of the engine chamber.
Therefore the rolling bearing for use in the idle pulley is demanded to have a high heat resistance, grease-sealing performance, and durability so that the rolling bearing withstands a very strict environment in which it is rotated at a high speed not less than 10000 rpm at a high temperature not less than 180° C.
As a grease composition preferable for the rolling bearing such as the rolling bearing for use in the electric auxiliary machine of a car which is used at high temperature and rotational speed, a grease composition is known. The grease composition contains 0.5 to 10 wt % of amide-based wax having the property of preventing the base oil from being oxidized and having a melting point not less than 80° C. The base oil has a kinematic viscosity of 20 to 150 mm2/second at 40° C. The grease composition contains the urea-based thickener at 5 to 30 wt % for the entire grease composition (see patent document 1).
In recent years, owing to the strict use condition that the rolling bearing is rotated at a high-speed rotation at a high temperature, a peculiar peeling phenomenon occurs in an early stage of use with the rolling surface of the rolling bearing turning into white in its texture, which has become a problem.
Unlike peeling which occurs in the inside of the rolling surface owing to metal fatigue, the peculiar peeling phenomenon occurs in the rolling surface at a comparatively shallow portion thereof with the rolling surface turning into white in its texture. That is, the peeling phenomenon is a hydrogen brittleness-caused destruction phenomenon.
As a method of preventing the occurrence of the peculiar peeling phenomenon which occurs in the early stage of use with the rolling surface turning into white in its texture, a method of adding a passivating agent to a grease composition is known (see patent document 2). A method of adding bismuth dithiocarbamate thereto is also known (see patent document 3).
But in recent years, at a high temperature, motors of the electric parts and the auxiliary machines of the car and those of the industrial machines are subjected to a frequent change of high-speed drive—sudden deceleration drive—sudden acceleration drive—sudden stop. Thus the use conditions of the rolling bearing have become increasingly strict. Therefore the method of adding the passivating agent to the grease composition and the method of adding the bismuth dithiocarbamate thereto are insufficient for preventing the occurrence of the peeling phenomenon.
Patent document 1: Japanese Patent Application Laid-Open No. 2003-105366
Patent document 2: Japanese Patent Application Laid-Open No. 3-210394
Patent document 3: Japanese Patent Application Laid-Open No. 2005-42102